1. Field of the Invention
The present invention relates to an image forming apparatus for forming an image by development, and relates to a control method for the image forming apparatus.
2. Description of the Related Art
In a conventional image forming apparatus for electrophotographic or electrostatic image formation, development is performed generally by using an image carrier (photosensitive drum) carrying thereon an electrostatic latent image and a developer carrier (developing sleeve) carrying a developer on it. At the time of development, a developing bias voltage is applied to the developing sleeve to generate an electric field between the sleeve and the photosensitive drum.
As the developing bias voltage, there is used for example a voltage having a DC (direct current) component on which an AC (alternating current) component is superimposed as shown in FIG. 18. Specifically, the bias voltage in FIG. 18 (referred to as the rectangular bias voltage in this specification) includes the AC component of a rectangular waveform having a frequency of about 2 kHz (corresponding to a half cycle of about 250 μsec) and a peak-to-peak voltage (Vp-p) of about 2 kV.
For a two-component development using a two-component developer comprised of toner and carrier, a technique has been proposed of using a developing bias voltage having a DC component on which an AC component is intermittently superimposed as shown in FIG. 19. The developing bias voltage in FIG. 19 (referred to as the blank pulse (BP) bias voltage in this specification) includes pulsatory parts and quiescent parts (blank parts).
With regard to waveforms of AC bias voltage (AC component of developing bias voltage), a variety of techniques have been proposed (see for example Japanese Laid-open Patent Publications Nos. 2001-194876, 2001-117332, and 2001-125348).
However, the prior art entails the following problems.
By using the developing bias voltage of FIG. 19 instead of the bias voltage shown in FIG. 18, the development ability can be improved. However, the pulsatory parts of the bias voltage must have a frequency not less than 4 kHz to eliminate coarseness of a highlight portion of a developed image. Besides, to attain a desired image density, the pulsatory parts of the bias voltage must have a peak-to-peak voltage Vp-p not less than about 2 kV as with a rectangular bias voltage shown in FIG. 18. The AC component (AC bias voltage) superimposed on the DC component shown in FIGS. 18 and 19 contributes to the improvement of development ability, but if the AC bias voltage is excessive, it causes image defects such as ring marks which are caused by foreign matter intruded into the interior of the developing device.
In an image forming apparatus, a predetermined gap (hereinafter referred to as the S-D gap) is defined between the developing sleeve and the photosensitive drum. To maintain the S-D gap, stop rollers are disposed at ends of the developing sleeve such as to abut against a surface of the photosensitive drum. Alternatively, a spacer is fixed between the rotary shaft of the developing sleeve and that of the photosensitive drum. The development ability is improved by a narrow S-D gap, but image defects are liable to occur when the S-D gap is narrow, as with a case where the AC bias voltage is excessively large. On the other hand, a wide S-D gap results in a reduction in development ability.
With a variation in diameter of the stop rollers or a variation in dimension of the spacer, the S-D gap varies between image forming apparatuses, which poses a problem. The S-D gap has a target value in design, i.e., a nominal value, which is extremely small in the order of, e.g., 300 μm and can vary about plus or minus 20% due to component part tolerances, etc. When the S-D gap decreases to, e.g., 240 μm from the nominal value of 300 μm, a high developing bias voltage is applied to the narrow S-D gap, and the resultant electric field in the S-D gap becomes extremely high.
When conductive foreign matters intrude into the interior of the developing device, the S-D gap becomes narrow at a part of the developing sleeve which faces the photosensitive drum and into which foreign matters intrude. At that sleeve part, there occurs abnormal discharge causing a problem that ring-like spots are formed on the image and the resultant image quality is largely impaired. When the S-D gap is wide, on the other hand, a problem is posed that the development ability is lowered as described above.
In single-component jumping development using a single-component developer comprised of toner including a magnetic material, the development is performed based on a phenomenon that a developer flies in an electric field formed between the developing sleeve and the photosensitive drum. For the single-component jumping development, a developing bias voltage comprised of a DC component and an AC component of rectangular wave superimposed thereon is mainly used. At the development, an ear of the developer in the developing sleeve is kept out of contact with the photosensitive drum, and the developer is caused to fly in the electric field generated by a potential difference between potential on the photosensitive drum for latent image formation and potential on the developing sleeve.
However, the electric field for the single-component jumping development varies depending on the distance of the S-D gap and the resultant image density varies. When the S-D gap dynamically varies, the image density becomes low at a part where the S-D gap is wide and becomes high at a part where the S-D gap is narrow, resulting in undesired bands of light and shade appearing on the image.